Many machines use planetary gear trains as part of their transmission or for other purposes. These gear trains or gear assemblies often include a carrier subassembly to which various components are attached such as the carrier, planetary axle, planetary gear and bearings. The inner cylindrical surfaces of the bores as well as the annular shaped thrust surface that surrounds the perimeter of some bores need to be tightly toleranced to provide the precision and support for the gear train to work properly. An interference fit is often providing between the inner surface of a bore and various components such as the planetary axle. Over time in operation, these surfaces may wear resulting in a loss of the capacity of the joint to work properly, requiring machine maintenance.
Furthermore, it is important that the gear train be assembled with great precision. When inserting a planetary axle into the carrier, the axle will sometimes become misaligned, causing the axle to hang up or jam in a crooked manner into a bore of the carrier. If not caught in inspection, this may lead to rework later on when the problem is discovered, which is undesirable. Regardless of when the problem is discovered, loss time and cost are incurred when fixing the problem.
FIG. 1 is a perspective view of a planetary gear train carrier member 102 as is known in the art. Upper large bores 104 and lower smaller bores 106 are provided that require precision machined inner cylindrical surfaces 108. Annular shaped thrust surfaces 110 that surround the perimeter of the lower bores 106 must also be precision machined. As already mentioned, these surfaces may become worn, requiring rework to enable the continued use of the carrier member. The large central bore 112 is for the shaft of the sun gear and the small core out holes 114 between the large upper bores 104 that surround the large central bore 112 are present to remove thick material portions of the carrier member 102, which may lead to undesirable manufacturing problems associated with sand casting such as sinks or porosity. The upper deck 116 is attached to the lower deck 118 by four standoffs 120 or rib like structures. As can be appreciated by looking at FIG. 1, the standoffs 120 form thick sections of the carrier member 102, necessitating the use of the core out holes 114. The upper deck 116, lower deck 118, skirt 122 and standoffs 120 define an interior annular groove 124 for the carrier member 102. The rim 126 of the carrier member 102 defines a plurality of mounting holes 128. Only two are shown for enhanced clarity, but it is to be understood that the array of mounting holes may extend about the periphery of the carrier member. Of features of the carrier member are also not shown for brevity.
FIG. 2 is a cross-sectional view of the planetary gear train 100 that is known in the art, showing a first step used in assembling the gear train 100 as the axle 130 is inserted through the upper large bore 104 of the carrier member 102 and begins to pass through the bore 132 of a bearing 134. As shown, the planetary gear 136 and bearings 134, 134′ are disposed between the upper deck 116 and lower deck 118 of the carrier member 102. The two bearings 134, 134′ have been inserted into the central aperture 138 of the planetary gear 136 and each bearing includes an inner race that also defines the central bore 132. The central bore 132 of the bearings 134, the central aperture 138 of the planetary gear 136, and the upper and lower bores 104, 106 of the carrier member 102 are all shown aligned concentrically, but in actuality, there may be misalignment between the bores of the carrier and those of the bearings as there is no positive method for alignment yet as the planetary gear and bearings are merely slid between the upper and lower deck before the insertion or pressing operation for the axle begins. In other words, the gear and the bearings are free to float relative to the carrier member along a radial direction R at this stage of the assembly process. As shown, the shaft portion 140 of the axle 130 is beginning to be inserted in to the bore 132 of the top bearing 134.
FIG. 3 shows step two of the assembly process depicted in FIG. 2 wherein the shaft 140 of the axle 130 has essentially passed through the bores 132, 132′ of the two inner bearing races arranged in concentric fashion relative to each other. The bearings 134, 134′ are adjacent each other along the longitudinal axis L of the rotational joint. As shown, the flange 142 of the axle 130 is about to engage the perimeter of the upper bore 104 of the carrier member 102 and the shaft 140 is about to engage the perimeter of the lower bore 106. It is at this point that misalignment may easily occur. Any of the embodiments discussed herein may use any suitable type of bearing, bushing, and/or method of lubrication that is already known or that will be devised in the art.
FIG. 4 illustrates the final step of assembling the axle 130 into the carrier member 102 wherein the flange 142 of the axle 130 is inserted into the upper bore 104 of the carrier member 102 and the shaft 140 of the axle 130 is inserted into the lower bore 106 of the carrier member 102. These insertion steps are usually accomplished using a pressing operation to overcome the friction inherent when press fitting one member into the aperture of another member. It is to be understood that that assembly operation may be completed as many times as needed. For example, four planetary gears may be provided using each of the pairs of large upper bores 104 and small lower bores 106 shown in FIG. 1. Though not shown in FIGS. 1 thru 4, a sun gear, located to the left of the planetary gear 136 in FIG. 4, would mesh with the planetary gear 136. Similarly, a ring gear would be disposed in the annular groove 124 shown to the right of the planetary gear 136 in FIG. 4 that meshes with the planetary gear.
One technique that has been used to complete this assembly process is visual alignment. The pressing operation used to insert the axle is performed until the flange of the axle is close to the large bore. The pressing operation is then stopped and realignment is performed visually. Then, the pressing operation is completed. This can be time consuming as an alignment window of only 1 mm is allowable using this method. It may take several iterations to use the visual alignment technique, which is undesirable.
Another technique has been referred to as the intentional misalignment technique. This technique involves forcing the components to one edge of the carrier until the main diameter of the planetary axle contacts the large bore. Then, the axle is pressed into the bearing races until the larger shoulder grounds into the top face of the carrier and deflects the top face. Next, the pressure is removed, allowing the deflected face of the upper deck to rebound resulting in the bearing and gear mass hanging suspended by the shoulder of the axle. Finally, the components are forced to the bore centerline. This causes the leading shoulder edge to drop into the large bore while the rest of the components are centered properly. Disadvantages of this method include increased assembly time, possible pinch points and damage to the face of the carrier when intentionally deflecting it using a press.